IXPE and XL-Calibur multi-mission spectropolarimetric fitting

X-ray polarimetry can be effectively used to examine the geometry and physical mechanisms of the innermost regions of compact X-ray sources. IXPE and XL-Calibur are both X-ray polarimetry missions operating in complimentary energy ranges, 2-8 keV and 15-75 keV respectively. IXPE functions as a photoelectric effect polarimeter, while XL-Calibur is a scattering polarimeter. Though these missions function via different physical mechanisms, the observed data for each is stored as Stokes parameters that can be easily compared. A simultaneous fit is the ideal way to maximize the spectropolarimetric information that is extracted. For example, the thermal emission from an accreting stellar mass black hole dominates in the IXPE energy range, while the coronal emission dominates in the XL-Calibur energy range. Simultaneous fits using each mission’s dataset can thus improve constraints on such a system. The modulation factor is a key parameter determining the sensitivity of a polarimeter. In both missions, the modulation factor is correlated with another parameter that isn't energy. In general, spectropolarimetric fits are able to utilize the energy-dependence of the modulation factor to improve fitting. Adding another parameter that correlates with the modulation factor can further improve the polarization sensitivity. For IXPE, the modulation factor is determined by the ellipticity of the track image. For XL-Calibur, event selection can be done by making geometric cuts to divide data roughly be Compton scattering angle. In both cases, strategically binning the data by these parameters can improve the polarization sensitivity. In the case of XL-Calibur, there is an expected improvement of ~8% when accounting for this extra parameter. A similar improvement is expected for IXPE. In this presentation I will describe a method for multi-mission spectropolarimetric fitting that takes into account additional parameters. Realistic simulations will be used to illustrate the expected performance.